http://www.pgroup.com/products/community.htm
PGI has a community edition. I think this is a new offering, but I could be wrong.
Anyways, PGI is owned by nvidia these days. Their compilers have a strong focus on HPC, and have support for OpenACC and OpenMP (among lots of other stuff). My understanding is that their generated code is pretty good, and they have some interesting tools built in to help find you missed opportunities for optimizations which may improve things for other compilers as well.
The C compiler doesn't masquerade as GCC (like clang and icc do) or MSVC, so it could be a good portability check for your code. It supports most of C11, but AFAICT no atomics or thread-local storage (except through OpenACC/OpenMP). There is a C++ compiler, too.
They seem pretty relaxed about licensing. Unlike Intel where you have to manage licenses, PGI has a EULA you have to agree to, but after that they trust you to obey it, which is refreshing. You don't even have to register on their web site if you don't want to.